Digital subscriber line (DSL) technology permits high-speed data transmission and Internet connectivity over conventional copper telephone lines. Conventional POTS (“Plain Old Telephone Service”) telephone lines were constructed to transmit human voice signals having a frequency range of 0-3.4 kHz. This is only a small fraction, however, of the bandwidth that can be supported by a copper wire. DSL technology uses this excessive available bandwidth for high-speed transmission of data. One of the more commonly used forms of DSL is asymmetric DSL (ADSL). ADSL recognizes the fact that most Internet users receive (download) much more information than they send (upload), and reserves the majority of the available channels for downstream transmission. A typical ADSL system provides downstream bit rates of up to 9 Mbps and upstream bit rates of up to 1 Mbps.
In conventional DSL networks, a virtual circuit for data transmission is established between a customer premises equipment (CPE) modem and a central office (CO) modem. A virtual circuit is a communications link that appears to the user to be a dedicated point-to-point circuit. It is identified by two numbers: the VPI (virtual channel identifier) and VCI (virtual channel identifier). When a customer first installs and initializes a CPE modem, the first information that is needed is the permanent virtual circuit settings that the CPE modem will need to communicate with the CO modem. In known configurations, the VPI/VCI identifiers are either pre-configured in the modem or are provided to the user who must manually enter the numbers. It would be desirable if the CPE modem could automatically sense and determine the correct VPI/VCI settings upon initial installation and initialization of the modem.
In addition to establishing a virtual circuit between the CPE modem and CO modem, a protocol defining the rules for sending and receiving information over the circuit must be established. In the ISO (International Organization for Standards) model, which defines a layered division of communications between applications on two computers or hosts, this type of protocol is known as a data link layer (layer 2) protocol. The point-to-point protocol (PPP), as defined in RFC 1661, is a popular data link layer protocol for connecting hosts by phone line. PPP provides a method for transmitting data over serial point-to-point links and has three main components: a method for encapsulating data; a link control protocol (LCP) for establishing, configuring and testing the data link connection; and a family of network control protocols (NCPs) for establishing and configuring different network layer protocols.
A PPP frame or packet, once formed, is itself encapsulated or framed for transport over the DSL network. Two encapsulation methods are of particular importance to DSL: PPP over Ethernet (PPPoE) and PPP over ATM (PPPoA). The encapsulation type that will be used, as with the virtual circuit settings, is typically programmed in or must be manually entered in the CPE modem at initialization. It would be advantageous if the CPE modem could automatically detect the type of encapsulation being used on its associated DSL line.